Mont. Code Ann. § 27-32-301

If the court is satisfied by competent evidence of the sufficiency of the petition that the applicant is a citizen of the United States, that the applicant has been a citizen of the state of Montana for 1 year preceding the filing of the petition and of the county for more than 90 days preceding the filing of the petition, and that the applicant's date of birth is proven, then the court shall render judgment accordingly and that judgment must constitute conclusive evidence of the date of the birth of the applicant.